Mid-back chairs offer fuller back support, and with the right ergonomic design, can be sat in for four hours at a time or longer. High-end chairs in this category, such as the Herman Miller Aeron and the Steelcase Leap are comfortable for long periods. Some mid-back chairs in particular offer customization options that can allow for a headrest to be added. Executive or full-back chairs offer full back and head support. Many executive chairs are designed to be sat in for eight or more hours at a time. These are typically the most expensive office chairs. http://www.ukachairs.com/office-chair/